A wide variety of downhole tools are used in conventional subterranean drilling operations. Such tools include, for example, drill bits, enlargement tools such as hole openers and underreamers, rotary steerable tools, measurement and logging while drilling tools, drilling motors, and stabilizers. Some such tools, for example, including certain expandable reamers, have a complex outer geometry that can create dead flow zones during drilling. Cuttings in the upwardly travelling annular column of drilling fluid can build up on the outer surface of the downhole tool at these dead zones. The presence of the cuttings can then lead to excessive abrasion and even scoring of the tool body during drilling. This phenomenon is sometimes referred to in the art as “mud ringing”. The scoring in the tool body can be so significant that the outer body of the tool must be scrapped in order to prevent crack propagation and premature or even catastrophic tool failure in service.
Various attempts have been made to solve this problem. For example, fluid flow can be diverted from the central bore of the tool to the annular column via placing one or more nozzles along the length of the affected tool. While these nozzles tend to eliminate the dead zones by creating additional hydraulic flow around the tool, they are not without drawbacks. In particular, such a diversion of the drilling fluid reduces pressure at the drill bit, which can in turn reduce penetration rates. In operations that make use of a drilling motor, diverting drilling fluid also reduces the pressure in the motor, which again tends to reduce penetration rates.
Attempts have also been made to apply various wear resistant coatings to the susceptible areas of the outer tool surface. However, the application of these coatings can introduce other difficulties. For example, such wear resistant coatings can be costly and difficult to adhere to the tool body. These coatings are known to flake off during service which can result in rapid erosion of the parent metal. The application of a sufficient thickness of coating material also tends to change the geometry of the tool body. Such changes are commonly unacceptable and thus create the need for the removal of parent metal from the tool body, which can in turn compromise tool strength.
Hardbanding techniques have also been utilized (e.g., in lieu of wear resistant coatings). In one such technique, a hardbanding material (e.g., tungsten carbide particulate) is deposited in a weld puddle formed on the surface of the tool. While the use of these techniques can improve wear resistance, they also can increase the susceptibility of the tool to cracking in the weld zone. Such a susceptibility to cracking tends to limit the use of heart banding techniques in downhole applications.
Therefore a need exists for a downhole tool body that resists mud ringing without the need to apply a wear resistant coating or to divert drilling fluid. There also exists a need for a method for increasing the wear resistance of an outer surface on a downhole tool so as to reduce scoring and ringing caused by caking of mud and cuttings during drilling operations.